教科书是
A.原始文献B.一次文献C.二次文献D.三次文献E.零次情报
参考答案:D
解析:教科书是在前人研究的基础上归纳总结得到的经典知识,属于三次文献。
已知集合M={y|y=2x,x∈R},N={y|y=x2,x∈R},则M∩N等于( )
A.(0,+∞)
B.[0,+∞)
C.{2,4}
D.{(2,4),(4,16)}
A.Curbing high-level corruption.
B.Fighting organized crime.
C.Investigating convictions of criminals.
D.Surveying the threats to national security.